1. Field of the Invention
This invention generally relates to an earphone and more particularly, is directed to a compact earphone which is adapted to be engaged with and attached to a cavum of a concave portion in an auricle during operation.
2. Description of the Prior Art
The low frequency range reproduction limit of a prior art open air-type earphone is mainly determined by a compliance Cmd (reciprocal number of stiffness S) of a vibration system and an equivalent mass Md of the vibration system. In this case, the response in the low frequency range in which the lowest resonance frequency is lower than f.sub.o is lowered. In general, the resonance frequency f.sub.o is substantially given as ##EQU1## In order to make the resonance frequency f.sub.o low, the compliance Cmd (cm/dyne) of the vibration system must be made high and/or the equivalent mass Md (gram) thereof must be increased. There is, however, a limit in increasing the compliance Cmd. Further, if the equivalent mass Md of the vibration system is increased, the auditory sensitivity becomes low and the acoustic characteristic in the high frequency range is deteriorated etc. Accordingly, there is naturally a limit in increasing the equivalent mass Md.
As is known, since there is a tendency that the lowest resonance frequency f.sub.o becomes higher as the diameter of the speaker unit becomes smaller, it is recognized that reproduced sound in a low frequency range is not heard by those who use an earphone of small diameter.
For instance, in the earphone which is disclosed in European patent publication No. 0064553, a magnetic circuit is formed of a magnet of a disc-shape, a cylindrical yoke and a plate surrounding the magnet. A diaphragm is coupled to a voice coil that is inserted into the air gap of the magnetic circuit and is incorporated in the front portion thereof to form a speaker unit. The outer peripheral portion of the speaker unit is mounted to a housing. At the rear surface of through-holes formed through the outer peripheral portion of the speaker unit, there is provided a samping layer and through-holes are formed through the housing to thereby control the frequency characteristic.
In other words, the acoustic circuit of this prior structure is expressed by a series circuit formed of an equivalent mass Md of the vibration system, a compliance Cmd thereof, an acoustic resistance Rd, a signal source Vs and an acoustic resistance Ra provided by the damping layer or the like, and a parallel circuit connected to the series circuit and formed of an acoustic resistance Rb by the through-hole of the housing and the compliance Cb by the back cavity.
However, since the acoustic resistance Rb provided by the through-holes of the housing is neglibibly small as compared with the acoustic resistance Ra of the damping layer or the like and the compliance Cb can be also neglected substantially, the effects of the acoustic resistance Rb and the compliance Cb constituting the parallel circuit are small, and thus the acoustic circuit substantially becomes a series resonance circuit formed of the equivalent mass Md, the compliance Cmd and the acoustic resonances Rd and Ra. Accordingly, the lowest resonance frequency f.sub.o is substantially given as ##EQU2## and it becomes difficult to set the frequency f.sub.o low.
Further, if the acoustic resistance Ra is small, a peak of acoustic characteristic is produced near the lowest resonance frequency f.sub.o so that only the nearby sound is emphasized and becomes a resonant sound, whereas if the acoustic resistance Ra is large, the acoustic characteristic is lowered from the frequency higher than the lowest resonance frequency f.sub.o so that the reproduction of low frequency range becomes insufficient.